ABSTRACT Retinal diseases are a leading cause of blindness, tremendously impacting patients and society. A root cause of poor vision is death of the photoreceptor (PR) cell, which primarily results from disruption of the normal homeostatic interaction between these cells and the underlying retinal pigment epithelium (RPE). Preserving PR viability and function remains a critical unmet medical need. Photoreceptor cells have the highest oxygen consumption in the body. The choroidal vasculature supplies this demand through the RPE ? a process that requires close apposition and intimate interaction. Periods of disrupted photoreceptor-RPE homeostasis might be expected to result in marked and rapid PR cell death. However, these cells can survive several days of reduced RPE nutritional support, resulting in the clinical window of opportunity for treating retinal disease. Currently there are no therapeutic options to maintain PR cell viability or slow the rate of cell death to extend this treatment window. In experimental retinal detachments (RD), a validated model of altered photoreceptor-RPE homeostasis, we have found activation of both survival and death genes and pathways. Examples of the former include the release of protective cytokines and activation of autophagy; whereas cell death occurs primarily through Fas-mediated apoptosis. A major gap in our knowledge is that we do not know the upstream activators of these cytoprotective and cytodestructive pathways. Our preliminary data strongly point towards the detachment-induced activation of hypoxia-inducible factors and microglia as playing a key role in these processes. Our central (working) hypothesis is that the temporal regulation of HIF and microglia represent key intrinsic and extrinsic influences that modulate Fas receptor-mediated apoptosis and the switch from PR survival to death. In Specific Aim 1 we will define the temporal regulation of HIF-1? and HIF-2? and their effect on Fas-mediated apoptosis of PRs. We propose that HIF-1? is activated early and is protective of PRs, but that, with time, there is a shift to HIF-2? activity, which is deleterious to PRs. As a corollary to this, we will test the hypothesis that one method by which HIFs exert their protective effect on PRs is by regulating the level of stress kinase activation. In Specific Aim 2 we will examine the role of microglia on the regulation of Fas-mediated PR death following retinal detachment. We hypothesize that early after RD, microglia are attracted to the outer retina by the release of chemotactic factors (ex: soluble CX3CL1 and CCL2) and proliferate. Initially, the microglia provide protective stimuli (ex: IL-6), but then shift phenotype during the evolving response to RD, with decreased production of protective factors and increased production of detrimental factors (ex: FasL and TNF?) accelerating PR death. The work proposed in this grant will provide a critical understanding of the molecular biology regulating the control of PR cell survival. It is only through the delineation of these fundamental processes that we will be able to develop targeted therapeutics to keep these cells alive.